The present invention relates to a ladder type electric filter having series resonators and parallel resonators housed in a casing and designed to form a unit filter circuit to be mounted on a printed-circuit board. Such an electric filter can suitably be used for telecommunications equipment such as a portable wireless telephone or an automobile telephone.
A ladder type electric filter comprises series resonators and parallel resonators, in which one of the electrodes of each series resonator is connected to an input terminal and one of the electrodes of each parallel resonator is connected to a ground whereas the other electrode of each of the series and parallel resonators is connected to an output terminal of the filter. Ladder type electric filters of the known type include those comprising square series and parallel resonators having edges of about 5 mm and designed to operate in a contour oscillation mode at a center frequency of 455 KHz and those comprising circular series and parallel resonators having a diameter of about 5 mm and designed to operate in a radial oscillation mode also at a center frequency of 455 KHz.
In the current trend of reducing the weight and size of every piece of radio telecommunications equipment, ladder type electric filters to be used in filter circuits are required be as low as 0.01 mm in thickness. On the other hands, conventional ladder type electric filters have a multilayer or superimposed structure of parallel resonators and series resonators, and the height of such filters is inevitably large as each filter comprises a casing and terminal plates that significantly contribute to the overall height of arrangement, making it rather difficult for the filter to become thin.
In an attempt to meet the requirement of low profile, as shown in FIG. 1 of the accompanying drawings, there has been proposed an arrangement where a pair of square parallel resonators P1 and P2 designed to operate in a contour oscillation mode are vertically disposed in two layers with the lower surface-electrode of the upper resonator P2 and the upper surface-electrode of lower resonator P1 connected with each other by way of a grounded terminal plate 1, and a pair of oblong series resonators S1 and S2 designed to operate in a longitudinal oscillation mode are disposed side by side on the upper parallel resonator P2 with an insulating plate 2 arranged therebetween such that the lower surface-electrodes of the series resonators S1 and S2 are electrically connected to the lower surface-electrode of the lowermost parallel resonator P1 by way of a connecting terminal plate 3 while the upper surface-electrode of the series resonator S1 is connected to an input terminal plate 4 and the upper surface-electrode of the series resonator S2 is connected to the upper surface-electrode of the parallel resonator P2 by way of an output terminal plate 5. All the above mentioned components are contained in a casing provided with an opening, which is hermetically sealed by an insulating shield plate having slits, through which the connecting legs 1a, 4a and 5a of the grounded, input and output terminal plates 1, 4 and 5 are extended outwardly and connected to respective external electric circuits. With this arrangement, the filter can be made to show a rather low profile because the series resonators S1 and S2 are juxtaposed on the same level and the height of not two but a single series resonator is added to the overall height of the filter.
With the above described arrangement, the connecting terminal plate 3 to be connected to the series resonators S1 and S2 is provided with a pair of protuberances or raised connecting sections 3.sub.s1 and 3.sub.s2 in alignment with the respective centers of the series resonators S1 and S2. Additionally, the connecting terminal plate 3, the output terminal plate 5 and a connecting terminal plate 6 to be connected to the parallel resonators P1 and P2 are provided with respective raised connecting sections 3.sub.p1, 5.sub.P2 and 6.sub.P1 in alignment with the centers of the electrodes of the parallel resonators P1 and P2. In the words, while the raised connecting sections 3.sub.s1 and 3.sub.s2 of the connecting terminal plate 3 are separated from each other and arranged side by side, the raised connecting sections 3.sub.p1, 5.sub.P2 and 6.sub.p1 of the terminal plates 3, 5 and 6 to be connected to the parallel resonators P1 and P2 are positioned on the center of the latter.
In a conventional ladder type electric filter having a configuration as described above, the series resonators S1 and S2 are made shorter than the parallel resonators P1 and P2 because the series resonators S1 and S2 need to have respective lengths with which their resonant frequencies correspond to respective given the frequencies in a longitudinal oscillation mode whereas the parallel resonators P1 and P2 need to have a length with which their resonant frequencies correspond to respective given the frequencies in a contour oscillation mode. On the other hand, the raised connecting sections 3.sub.p1, 5.sub.P2 and 6.sub.P1 of the terminal plates 3, 5 and 6 are laterally aligned with the centers of the parallel resonators P1 and P2.
As a result and as shown in FIG. 2, when the components are assembled and housed in the casing, they are subjected to various pressures. More specifically, pressures p1 and p2 are applied to the connecting terminal 3 respectively by way of the raised connecting sections 3.sub.s1 and 3.sub.s2 that are held in close contact with the respective series resonators S1 and S2, while pressure p3 is applied to the center of the neighboring terminal plate 5 by way of the raised section 5.sub.P2 located at the middle of the raised sections 3.sub.s1 and 3.sub.s2 to make the terminal plates warp with the center of the raised connecting section 5.sub.P2 acting as the fulcrum. Consequently, the points at which the series resonators S1 and S2 contact respectively with the raised connecting sections 3.sub.s1 and 3.sub.s2 are laterally shifted. While the contact points of the raised connecting sections 3.sub.s1 and 3.sub.s2 are so selected as to correspond to the respective nodes (centers) of oscillation of the series resonators S1 and S2, the resonance characteristic of the filter is inevitably varied as the contact points are shifted.
Additionally, as illustrated in FIG. 3 schematically in a somewhat exaggerated manner, as the contact point of the raised connecting section (3.sub.s1 ; 3.sub.s2) of the terminal plate 3 is positionally shifted relative to the center (fulcrum) O of each of the series resonators S1 and S2, the latter are subjected to a torque m as indicated by an arrow in FIG. 3 and tend to become inclined. As the series resonators S1 and S2 are inclined, their peripheral areas abut the casing 7 in an uneven manner to alter that the resonance characteristic of the series resonators and hence that of the filter as a whole. While this problem may be alleviated by aligning the raised connecting sections 3.sub.s1 and 3.sub.s2 of the terminal plate 3 with the centers of the series resonators S1 and S2, respectively, then a similar problem arises between the parallel resonators P1 and P2 and the corresponding raised connecting sections of the terminal plates that are held in contact with the respective parallel resonators.
It is, therefore, an object of the present invention to provide an improved ladder type electric filter that is free from the above identified problems or disadvantages.